arc: Add support for signal frames for Linux targets

Implement functions needed to unwind signal frames on ARC Linux targets.

gdb/ChangeLog

	* arc-linux-tdep.c (arc_linux_sc_reg_offsets): New static variable.
	(arc_linux_is_sigtramp): New function.
	(arc_linux_sigcontext_addr): Likewise.
	(arc_linux_init_osabi): Use them.